The effective range of a mobile device, such as a cordless handset, from its Base Station is limited by its transmission power and by the receiver sensitivity of the mobile device and the Base Station. Wireless Private Branch Exchange (WPBX) systems address this limitation by using more than one Base Station (BS). The area that a Base Station covers is called a cell. In the main, hereinafter, mobile units (devices) that are cordless (telephone) handsets are discussed.
In a WPBX, the Base Stations are interconnected in order to allow handsets that are in different cells to communicate with one another. When a handset moves from one cell to another during a call, the handoff (or handover) of communication from one Base Station to another Base Station enables uninterrupted communication. A central unit that is usually called the “Switch” is connected to all the Base Stations. The Switch controls the operation of the system, routes the call to Base Stations and to Gateways, which connect the WPBX to external communication systems. The transmission power of a cordless handset in the WPBX is usually lower than the transmission power of the handset of a standard cellular system, which results in a WPBX for cordless handsets having much smaller cells (referred to as mini-cells, or micro-cells or picocells) than the cells of a standard cellular system.
Some cordless handsets use communication protocols that are also used in cellular system, but they transmit in a lower power than a mobile (cellular) handset. For examples protocols in use are GSM and IS-136. According to these protocols the handoff between cells is performed by collaboration of the cordless handset, the Base Stations and the Switch. These handsets can connect to the WPBX when they are in its coverage area, and can also connect to any other cellular system that supports the communication protocol that they are using.
Some handsets use communication protocols that were designed especially to allow communication with WPBX. Some examples are DECT, CT-2, PAC, and PACS. The handset is usually a dedicated handset that is used only in the area covered by the WBPX.
Some handsets have dual mode support. For example a handset may communicate with the WPBX using DECT, and may allow communication with other cellular systems using GSM.
Some WPBXs use standard cordless handsets. These handsets have no special mechanism to support the handoff between cells. In these systems the Switch and the Base Stations perform the handoff, and the handset is not aware of (does not participate actively in) the handoff process. When a standard cordless handset moves from one cell to another the Switch routes the call to another cell. Since cordless phones use “simple” protocols, for example an analog fixed transmission, when the call is routed to the new cell, the cordless phone automatically will receive it.
During the last years short-range communication protocols have become much more complicated. Very low power is used in order to allow many systems to operate in close vicinity. Complex transmissions methods like frequency hopping and spread spectrum are used in order to overcome interference, and improve the communication quality. Digital communication methods are used allowing communication of data and voice on the same system. Error correction encoders are used in order to improve reliability. Security and privacy of the communication is improved with the use of Digital authentication and encryption.
Short-range communication systems are used for many purposes. A growing trend for short-range communication usage is Personal Area Network (PAN) devices and applications, among such is the “all in one handset” and personal data devices. Such type of handset supports standard cellular communication, and also has the ability to communicate with personal area network devices that are in its near vicinity, using short-range communication. Some PAN short-range communication standards were not designed to allow mobility, i.e. they were not designed to allow handoff in between Base Stations in general and during an active session in particular. This limits a session via such device to be linked to a single Base Station and therefore to very limited area.
The “Bluetooth” standard is a short-range wireless communication standard that has many uses for voice applications and telephony (e.g. cordless phone, wireless headsets) and also for data applications (laptop to personal computer communication, wireless local area network Gateways etc.). The Bluetooth wireless technology is implemented using a universal radio interface in the 2.45 GHz frequency band that enables portable electronic devices to connect and communicate wirelessly via short-range, ad hoc networks. Each unit can simultaneously communicate with up to seven other units per piconet. Moreover, each unit can simultaneously belong to several piconets.
Bluetooth connection is planned to be standard feature in future cellular handsets, Personal Digital Assistants (PDAs), Palmtop and Laptop computers. The Bluetooth standard does not support mobility between Base Stations, since it was primarily designed for short-range communication as a cable replacement. A cellular handset with Bluetooth wireless technology will be able to operate as a cordless phone, but only in the near vicinity of a single Base Station. The same limitation applies to mobile personal data devices such as PDA's and mobile computers.